Technical Field
The present disclosure relates to a control device for a power converter, in particular a power-factor-correction (PFC) regulator, with interleaved stages, i.e., including two or more converter stages operating with a suitable mutual phase offset; the present disclosure further regards a corresponding switching regulator and a corresponding control method.
Description of the Related Art
Switched-mode power supplies are known, designed to convert a quantity received at an input, for example an AC voltage coming from the electrical mains supply, into a regulated output quantity, for example a DC voltage, for supply of an electrical load.
Such power supplies have generally to meet stringent requirements as regards the corresponding electrical performance; for example, they have to guarantee a high quality factor and a substantially unitary power factor.
For this reason, it is generally envisaged the use, in an input stage of the power supply, of a switching regulator, the so-called “PFC regulator”, controlled by a suitable control device for regulating the power factor in drawing from the power grid.
In particular, in applications that entail drawing of a considerable power, for example of about 1 kW, for instance for flat screen television (flat TVs) or for industrial applications, the use has been proposed of a PFC regulator having a number of interleaved converter stages, operating with a suitable phase offset. It may be shown in fact that use of a number of interleaved converter stages enables an increase of the power level that may be reached as compared to the use of a single converter stage.
A PFC regulator 1 is depicted by way of example in FIG. 1; PFC regulator 1 is controlled by a corresponding control device 2 and comprises two converter stages 3 (not described in detail herein), interleaved and operating in this case in phase opposition, i.e., with a phase offset of 180°.
The PFC regulator 1 has: an input terminal IN, present on which is an input voltage VIN, generated by a rectifier stage 5 starting from an AC line voltage VAC, for example supplied by a supply line 6 from the electrical grid; and an output terminal OUT, to which a charge-storage element 7 is connected, in particular a capacitor, present on which is an output voltage VOUT, for example a DC voltage, which is regulated at a desired value.
In a way not described in detail herein and illustrated in FIG. 1, each converter stage 3 comprises at least one inductive element and a switch element, which is operatively coupled to the inductive element and is controlled in pulse-width-modulation (PWM) switching by the control device 2, for cyclically storing energy in the inductive element in a first interval (for example, the so-called ON interval TON) of the switching period, and for releasing the energy stored to the charge-storage element 7 in a second interval (in the example, the so-called OFF interval TOFF) of the switching period.
The control device 2 is made as an integrated circuit, and has a package and corresponding input and output pins, and may be mounted on a same printed circuit board (PCB) with the circuit components forming the PFC regulator 1.
In a way not described in detail herein, the control device 2 supplies command signals to the switch elements of the converter stages 3, and receives feedback signals from the same converter stages 3, to provide regulation of the output voltage Vout via an appropriate control loop.
The purpose of the control device 2 is to obtain in the inductive elements of the converter stages 3 currents that are phase-shifted by the desired phase offset (in the example, by 180°). In this regard, FIG. 2 shows the desired plot of the currents in the inductive elements of the converter stages 3, designated by IL1 and IL2, which are phase-offset by 180°.
Achieving this purpose is not, however, altogether straightforward, given that the system works at a continuously variable switching frequency, as the working condition varies, for example as a result of: the variation of the line voltage VAC between successive line periods, or cycles; the presence of noise on the supply line 6; the variation of the power on the load; the presence of start-up and shut-down transients; or the presence of so-called “phase shedding” events (in order to optimize the overall efficiency of the system, individual stages may be started up or shut down to modify the number of active stages for adapting to the requirements of the load). Furthermore, the switching frequency may be variable also on account of possible tolerances and drifts in the electrical components.
There are several solutions that have so far been proposed to achieve the control objective referred to previously.
For instance, an open-loop-control approach of the master-slave type has been proposed. In this regard, there may be cited, for example:    L. Huber, B. T. Irving, and M. M. Jovanovic, “Open-loop control methods for Interleaved DCM/CCM Boundary Boost PFC converters”, IEEE Trans., Power Electron., July, 2008; and    L. Huber, B. T. Irving, C. Adragna, and M. M. Jovanovic, “Implementation of Open-loop-control for Interleaved DCM/CCM Boundary Boost PFC converters”, IEEE Applied Power Electronics Conf. APEC, February 2008.
In this solution, one converter stage operates in so-called transition mode (TM) while the other converter stage operates in discontinuous-conduction mode (DCM). This operating mode leads to a deterioration of the performance of the system. In DCM, there is in fact a reduction of the overall efficiency of the system and an increase of electromagnetic interference (EMI).
A further solution proposed envisages a closed-loop control based upon a phase-locked loop (PLL). In this regard, the following may, for example, be cited:    L. Huber, B. T. Irving, and M. M. Jovanovic, “Closed-loop control methods for Interleaved DCM/CCM Boundary Boost PFC converters”, Proc. IEEE Appl. Power Electron. Conf., February, 2009.
The so-called natural-interleaving technique belongs to this category described, for example, in:    B. Lu, “A novel control method for Interleaved Transition Mode PFC”, IEEE 2008—TI UCC28060.
In this solution, both of the converter stages operate as master, and the duration of the ON interval (TON) in the switching period of each stage is modulated on the basis of the relation of phase and frequency. In this way, transition-mode (TM) operation is guaranteed for both phases.
Since closed-loop methods require lowpass filtering, they are able to respond to disturbance and to transients in a relatively slow way. In some cases, a change of the working mode and a consequent loss of interleaving may cause a severe unbalancing between the currents of the individual phases.
A further solution proposed envisages a cross-coupled master-slave relationship; see for example:    H. Choi and L. Balogh, “A cross-coupled master-slave Interleaving Method for boundary conduction Mode PFC converters”, IEEE Trans. Power Electron., Oct. 20, 2012—Fairchild FAN9611.
In the above solution, the natural period of each phase is measured, and in the next cycle this period is used together with the so-called zero-crossing-detection (ZCD) signal, for determining the instant of turning-on of the switch element of the converter stages. Consequently, each phase may each time be master or slave, and the interleaving timing may be modified dynamically so that the solution is robust to transients.
The present Applicant has, however, realized that also this solution is not altogether advantageous, at least in that it does not guarantee a transition operating mode for both of the phases.